Unsupported firefox `null` values in json policy

[mrandybu]

If created policy file for firefox contains null value, firefox can't read it.

[olivergs]

Is this related with the issue #251, #263 in fc-admin?

This null values shouldn't happen as with the change you finally done the null values are just not sent.

If this is the case, please close the PR. If not, please tell me when this could happen to know what we are addressing.